Longer contact times increase cross-contamination of Enterobacter aerogenes from surfaces to food

Bacterial cross-contamination from surfaces to food can contribute to foodborne disease. The cross-contamination rate of Enterobacter aerogenes on household surfaces was evaluated by using scenarios that differed by surface type, food type, contact time (<1, 5, 30, and 300 s), and inoculum matrix (tryptic soy broth or peptone buffer). The surfaces used were stainless steel, tile, wood, and carpet. The food types were watermelon, bread, bread with butter, and gummy candy. Surfaces (25 cm²) were spot inoculated with 1 ml of inoculum and allowed to dry for 5 h, yielding an approximate concentration of 10⁷ CFU/surface. Foods (with a 16-cm² contact area) were dropped onto the surfaces from a height of 12.5 cm and left to rest as appropriate. Posttransfer, surfaces and foods were placed in sterile filter bags and homogenized or massaged, diluted, and plated on tryptic soy agar. The transfer rate was quantified as the log percent transfer from the surface to the food. Contact time, food, and surface type all had highly significant effects (P<0.000001) on the log percent transfer of bacteria. The inoculum matrix (tryptic soy broth or peptone buffer) also had a significant effect on transfer (P = 0.013), and most interaction terms were significant. More bacteria transferred to watermelon (~ 0.2 to 97%) than to any other food, while the least bacteria transferred to gummy candy (~ 0.1 to 62%). Transfer of bacteria to bread (~ 0.02 to 94%) was similar to transfer of bacteria to bread with butter (~ 0.02 to 82%), and these transfer rates under a given set of conditions were more variable than with watermelon and gummy candy.